Decorator

M3 · Wrappers: The Four SiblingsSession 6-7StructuralKotlin replaces itOutline

Intent. Keep the interface and add responsibilities, stackable at runtime.

In Kotlin. class Logged(private val inner: Repo) : Repo by inner – override what you intercept, delegate the rest.

Keeps the interface and adds responsibility, and it stacks. Its diagram is identical to Proxy’s – the difference is that a decorator adds behavior while a proxy controls access to the same behavior. Kotlin’s interface delegation makes a decorator a one-liner, which is why this is the wrapper you will actually reach for.

Structure

classDiagram
    class Component {
        <<interface>>
        +operation()
    }
    class ConcreteComponent {
        +operation()
    }
    class Decorator {
        #inner: Component
        +operation()
    }
    class LoggingDecorator {
        +operation()
    }
    class CachingDecorator {
        +operation()
    }
    Component <|.. ConcreteComponent
    Component <|.. Decorator
    Decorator o--> Component : wraps one
    Decorator <|-- LoggingDecorator
    Decorator <|-- CachingDecorator

Telling It Apart

Patterns whose structure looks the same but whose intent does not. See the Confusing Pairs reference.

• Proxy — Keep the interface and control access – lazy loading, caching, permissions, remoting.
• Adapter — Change an interface so two otherwise incompatible things can work together.
• Composite — Treat individual objects and compositions of objects uniformly.
• Chain of Responsibility — Pass a request along a chain until some handler takes it, and can stop it.